Process for fabricating a projection electron lithography mask and a removable reusable cover for use therein

ABSTRACT

A removable, reusable cover constructed such that its geometry matches the geometry of the active region of a projection electron lithography mask to be protected and does not etch in the plasma environment used to remove a photoresist. The cover protects the active region of the projection electron lithography mask, but does not contact the active region. A technique for fabricating a projection electron lithography mask utilizing the removable, reusable cover, where the geometry of the cover is matched to the geometry of an active region of the projection electron lithography mask to be protected. During fabrication of the projection electron lithography mask, the cover protects the active region of the projection electron lithography mask from the plasma environment, but does not contact the active region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for fabricating a projectionelectron lithography mask and a removable, reusable cover for usetherein, and more particularly, to a technique for fabricating aprojection electron lithography mask and a removable, reusable cover foruse therein, wherein the geometry of the cover is matched to thegeometry of an active region (membrane plus strut) of the projectionelectron lithography mask.

2. Description of the Related Art

Projection electron beam lithography, such as Scattering AngularLimitation Projection Electron Beam Lithography (SCALPEL™), utilizeselectron beam radiation projected onto a patterned mask to transfer animage of that pattern into a layer of energy sensitive material formedon a substrate. The image is developed and used in subsequent processingto form devices such as integrated circuits.

The SCALPEL™ mask has a membrane of a low atomic number material onwhich is formed a layer of high atomic number material. The layer ofhigh atomic number material has a pattern delineated therein. Both thelow atomic number membrane material and the high atomic number patternedlayer of material are transparent to the electrons projected thereon(i.e., electrons with an energy of about 100 keV). However, the lowatomic number membrane materials scatters the electrons weakly and atsmall angles. The high atomic number patterned layer of materialscatters the electrons strongly and at large angles. Thus, the electronstransmitted through the high atomic number patterned material have alarger scattering angle than the electrons transmitted through themembrane. This difference in scattering angle provides a contrastbetween the electrons transmitted through the membrane alone and theelectrons transmitted through the layer of patterned material formed onthe membrane.

This contrast is exploited to transfer an image of the pattern from themask and into a layer of energy sensitive material by using a back focalplane filter in the projection optics between the mask and the layer ofenergy sensitive material. The back focal plane filter has an aperturetherein. The weakly scattered electrons are transmitted through theaperture while the strongly scattered electrons are blocked by the backfocal plane filter. Thus, the image of the pattern defined in the weaklyscattered electrons is transmitted through the aperture and into thelayer of energy sensitive material.

FIG. 1 is a schematic diagram illustrating the concept of a conventionalSCALPEL™ system. A beam B of electrons 10 is directed towards ascattering mask 9 including a thin membrane 11 having a thicknessbetween about 1,000 Å and about 20,000 Å (0.1 μm and about 2 μm thick.)The membrane 11 is composed of a material which is virtually transparentto the electron beam B composed of electrons 10. That is to say thatelectrons 10 in beam B pass through membrane 11 freely in the absence ofany other object providing an obstruction to the path of electrons 10 inthe beam B as they pass from the source of the beam through the membrane11.

Formed on the side of the membrane 11 facing the beam 10, is a patternof higher atomic number, higher density scattering elements 12 toprovide a contrast mechanism that enables reproduction of the maskpattern at the target surface. The scattering elements 12 are patternedin the composite shape which is to be exposed upon a work piece 17(usually a silicon wafer) which is coated with e-beam sensitive resist,which as shown in FIG. 1 has been processed into pattern elements 18.The electrons 10 from the e-beam B which pass through the mask 9 areshown by beams 14 which pass through electromagnetic lens 15 whichfocuses the beams 14 through an aperture 16′ into an otherwise opaqueback focal plane filter 16. The aperture 16′ permits only electronsscattered at small angles to pass through to the work piece 17.

A conventional SCALPEL™ exposure tool is illustrated in FIG. 2. Theexposure tool 20 includes a source 22 (usually an electron gun), a maskstage 24, imaging optics 26, and a wafer stage 28. The mask stage 24 andthe wafer stage 28 are mounted to the top and bottom of a block ofaluminum, referred to as the metrology plate 30. The metrology plate 30,which is on the order of 3000 lbs., serves as a thermal and mechanicalstabilizer for the entire exposure tool 20.

FIG. 3 illustrates the conventional mask stage 24, the imaging optics26, and the wafer stage 28 in more detail. As illustrated in FIG. 3, thesource 22 outputs an electron beam, which is aligned and focused on alens C1 by a gun alignment deflector 40 and a shaping aperture 42. Theelectron beam is further focused on a lens C2 by a beam blankingdeflector 44, an illumination deflector 46, and blanking aperture 48.After passing through lens C2, the electron beam impinges on the mask 9and is focused on the wafer 17 utilizing lenses P1 and P2 and deflectorsP1 and P2 and a SCALPEL™ aperture 50.

The conventional SCALPEL™ mask 9 is formed by a process by which thehigher atomic number, higher density scattering elements 12 are formedfrom a polymeric film (or resist) that is spin-coated on the wafer 17 atselected locations. However, during the spin coating process, resistremains on the wafer 17 at undesired locations.

The unwanted regions are primarily located outside the active region(illustrated as element 60 in FIG. 4). Typically, the unwanted resistcovers alignment marks (illustrated as element 62 in FIG. 4) which arepatterned during the SCALPEL™ mask blank metal deposition process. Theremoval of the resist over the marks improves the ability to detect themarks by subsequent exposure of the mask, inspection and metrologytools. In addition to removing the resist from the marks, grounding padsin each comer of the metalized region are exposed so as to allow for apoint of contact for grounding the surface during direct write e-beamexposure. Therefore, a method is required to provide for the removal ofthe resist from the unwanted regions of the SCALPEL™ mask.

It is also desirable to selectively remove the resist from the unwantedregions of the SCALPEL™ mask and still maintain (a) cleanliness, (b)reusability, (c) feasibility in a production-type environment, and (d)add no adverse effects to the imaging resist characteristics (forexample, sensitivity, damage, etc.).

The conventional method of removing unwanted resist from the unwantedregions of the SCALPEL™ mask is to use a solvent in a standardradial-type removal. However, this technique is limited to removingresist radially from the edge of the SCALPEL™ mask and hence, the activeregion of the SCALPEL™ mask must be circular in shape.

Furthermore, the standard method of using a solvent to remove the resistfrom the edge of the wafer 17 in a radial fashion requires an additionalstep in the resist coat process, namely the step of dispensing a solvent(usually via a syringe or tube) over the wafer's edge. Its primarypurpose is to allow for the removal of excess resist buildup near theedge of the wafer 17. For SCALPEL™ masks with other than circular activeregions (such as the rectangular active region 60 of SCALPEL™ mask 9illustrated in FIG. 4 and 7), it is impossible to uncover alignmentmarks 62 using the standard method without loss of the resist coating inthe active region. FIG. 5 is a photograph showing a loss of resistcoverage in an active region on a 100 mm SCALPEL™ mask blank, using wetetch resist removal.

The topology of the alignment marks also is a problem in that thealignment marks are in the form of crosses and it is difficult(sometimes impossible) to remove resist trapped in the intersections ofthe alignment marks utilizing the standard wet solvent removal process.

SUMMARY OF THE INVENTION

The present invention solves these problems with conventional SCALPEL™masks and conventional techniques for removing unwanted resist outsideof the active region of the SCALPEL™ mask by providing a removable,reusable cover which is constructed such that the geometry of the covermatches the geometry of the active region of a projection electronlithography mask to be protected and does not deteriorate in the plasmaetching environment used to remove the resist. In particular, the coverof the present invention protects the active region from the plasmaetching environment of the projection electron lithography mask, butdoes not contact the active region.

The present invention is also directed to a technique for fabricating aprojection electron lithography mask utilizing the removable, reusablecover. During fabrication of the projection electron lithography mask,the cover is placed over the active region of the projection electronlithography mask to protect, but not contact, the active region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the concept of a SCALPEL™system.

FIG. 2 illustrates a conventional SCALPEL™ exposure tool.

FIG. 3 illustrates portions of the conventional SCALPEL™ exposure toolof FIG. 2 in more detail.

FIG. 4 illustrates a SCALPEL™ mask and wafer in more detail.

FIG. 5 illustrates a loss of the resist coating in the active regionusing conventional resist removal techniques.

FIG. 6 illustrates a general flowchart for fabricating a projectionelectron lithography mask.

FIG. 7 illustrates the cover of the present invention in a preferredembodiment.

FIG. 8 illustrates the process for fabricating a projection electronlithography mask of the present invention in a preferred embodiment.

FIG. 9 illustrates the result of the process of FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is generally directed to a removable, reusablecover constructed such that its geometry matches the geometry of theactive region of a projection electron lithography mask to be protectedand does not etch in the plasma environment used to remove thephotoresist. In a preferred embodiment, the cover protects the activeregion of the projection electron lithography mask, but does not contactthe active region.

The present invention is also generally directed to a technique forfabricating a projection electron lithography mask utilizing theremovable, reusable cover, wherein the geometry of the cover is matchedto the geometry of an active region of the projection electronlithography mask to be protected. In a preferred embodiment, duringfabrication of the projection electron lithography mask, the coverprotects the active region of the projection electron lithography maskfrom the plasma environment, but does not contact the active region.

FIG. 6 illustrates a general flowchart for removing resist fromnon-active regions of the mask utilizing the removable, reusable cover.First, in step 70, the cover, whose geometry is matched to the geometryof an active region of the projection electron lithography mask to beprotected is placed within a plasma etch unit (such as reactive ionetcher or RIE) such that the cover protects the active region of theprojection electron lithography mask, but does not contact the activeregion.

Next, in step 72, the plasma etch process is initiated in the plasmaetch unit to remove the unwanted resist outside the active region. Inthe plasma etch unit, oxygen or suitable etching gas for the resist isbrought into a plasma state between two electrodes. The gas and powerlevels are adjusted to create the desirable etch rate. In the presentembodiment, the unwanted resist is removed by dry gaseous plasma.

Given the RIE would remove the entire resist layer if left unprotected,the desired active region is protected by the cover 100. The cover 100does not etch itself in the gas/plasma environment used to etch theresist.

FIG. 7 illustrates a preferred embodiment of the cover 100 of thepresent invention in more detail. In FIG. 7, the active region of theSCALPEL™ mask 9 is facing upward. As illustrated, the cover 100 includestwo parts; a lower member 102, on which the membrane of the SCALPEL™mask 9 rests, and a upper portion 104 that lies on or above the SCALPEL™mask 9. The lower member 102 protects the back side surface of theSCALPEL™ mask 9 from direct contact with the RIE. The upper portion 104protects the active region of the SCALPEL™ mask 9 during the plasma etchstep 72.

In one embodiment of the present invention, the lower member 102contacts only at the perimeter of the surface of the SCALPEL™ mask andhas minimal contact with an edge of the SCALPEL™ mask 9 and alsoprovides for a stand-off height to allow for the evacuation of airduring the plasma etch step 72.

The membrane 11 of the SCALPEL™ mask 9 is a thin film structure. Trappedair could cause enough turbulence in the evacuation process that wouldultimately lead to breakage of the membrane 11. As a result, the lowermember 102 includes an air escape passage to allow the air to escape andprevent rupture of the membrane 11.

In a preferred embodiment, the lower member 102 is a support ring with anotch 106 therein, as illustrated in FIG. 7.

The upper portion 104 is dimensioned so as to allow maximum protectionof the resist coating in the active region of the SCALPEL™ mask 9. Theupper portion 104 can be dimensioned to allow for coverage of any shapeof active region. The upper portion 104 is vented to allow removal ofair during a vacuum pump-down step of the RIE. The upper portion 104does not contact the active region of the SCALPEL™ mask 9. FIG. 4 and 7illustrate an outline 108 of the cover 100, which completely covers theactive region of the SCALPEL™ mask 9, but does not contact the activeregion.

In a preferred embodiment, the upper portion 104 includes a rectangularprotective portion 110 and a handle 112, as illustrated in FIG. 7. Thehandle 112 aids in the placement and removal of the upper portion 104 byenabling self-alignment of the cover 100 and the active region 60. Thehandle 112 further includes a linear portion 114 and a semicircularportion 116 as illustrated in FIG. 7 to further facilitate placement andremoval of the cover 100.

In a preferred embodiment, the lower portion 102 and the upper portion104 are made of any material which is not removed in the plasma etchingenvironment used to etch the resist. In a preferred embodiment thematerial is aluminum because aluminum is not easily oxidized. In anotherpreferred embodiment, the lower portion 102 and the upper portion 104are made of Ultum which is a material compatible with the oxygen etchprocess.

FIG. 8 illustrates a preferred embodiment of the process for fabricatinga projection electron lithography mask such as the SCALPEL™ maskillustrated in FIG. 7, of the present invention in more detail. First,in step 80, the lower portion 102 is placed within the reactive ionetcher. Then, in step 82, the SCALPEL™ mask 9 is placed onto the lowerportion 102. Next, in step 84, the upper portion 104 is oriented overthe active region of the SCALPEL™ mask 9. Finally, in step 86, theplasma etch process is initiated to remove the unwanted resist outsidethe active region. The end result of the fabrication process isillustrated in FIG. 9, which shows complete resist coverage of theactive region on a 100 mm SCALPEL™ mask blank, using dry etch resistremoval.

The preferred embodiment of the present invention illustrated in FIG. 7specifically describes a rectangular active region 60 and a rectangularshape cover 100. However, the invention of the present applicationshould not be limited to such a structure. In particular, one aspect ofthe present invention is that the geometry of the protective portion 110of the cover 100 should be matched the geometry of the active region 60of the SCALPEL™ mask to be protected. It is another aspect of thepresent invention that the cover is not etched in the plasma environmentused to remove the photoresist. This means that the cover will protectthe action region 60 and also be reusable for other active regions ofthe same or similar shape. Another aspect of the present invention isthat the protective portion 110 contacts the SCALPEL™ mask 9 at aperimeter of the protective portion 110, but does not contact the activeregion 60.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A removable, reusable cover for protecting anactive region of a projection electron lithography mask, wherein ageometry of said cover substantially matches a geometry of the activeregion, such that said cover protects the active region, but does notcontact the active region during plasma etching, wherein said cover ismade of a material that is not substantially removed by the plasmaetching.
 2. The removable, reusable cover of claim 1, wherein theprojection electron lithography mask is a SCALPEL™ mask.
 3. Theremovable, reusable cover of claim 1, wherein said cover is made ofaluminum or ultum.
 4. The removable, reusable cover of claim 1, whereinsaid cover permits air escape to prevent rupture of the projectionelectron lithography mask.
 5. The removable, reusable cover of claim 1,wherein the plasma etching utilizes dry gaseous plasma.
 6. Theremovable, reusable cover of claim 1, wherein the plasma etching is usedto remove resist from alignment marks in the projection electronlithography mask.
 7. The removable, reusable cover of claim 1, whereinsaid cover includes an upper portion and a lower portion, wherein saidlower portion is placed below the projection electron lithography maskand said upper portion is placed above the projection electronlithography mask.
 8. The removable, reusable cover of claim 7, whereinsaid upper portion includes a handle for facilitating placement andalignment of said upper portion, said lower portion and the projectionelectron lithography mask, and a protective portion that protects theactive region of the projection electron lithography mask.
 9. Theremovable, reusable cover of claim 8, wherein said protective portionincludes a edge which contacts an outline on the projection electronlithography mask, wherein the edge is the only area of said upperportion to contact the projection electron lithography mask.
 10. Theremovable, reusable cover of claim 8, wherein said lower portion andsaid handle are shaped to facilitate placement and alignment of saidupper portion, said lower portion and the projection electronlithography mask.
 11. A process for fabricating a projection electronlithography mask, comprising: placing the projection electronlithography mask and a removable, reusable cover in a plasma etchenvironment, wherein a geometry of the cover substantially matches ageometry of an active region of the projection electron lithographymask, such that the cover protects the active region, but does notcontact the active region; and etching the projection electronlithography mask to remove resist outside the active region, wherein thecover is made of a material that is not substantially removed duringetching.
 12. The process of claim 11, wherein the projection electronlithography mask is a SCALPEL™ mask.
 13. The process of claim 11,wherein the cover is made of aluminum or ultum.
 14. The process of claim11, wherein the cover permits air escape to prevent rupture of theprojection electron lithography mask.
 15. The process of claim 11,wherein said etching step utilizes dry gaseous plasma.
 16. The processof claim 11, wherein said etching step removes resist from alignmentmarks in the projection electron lithography mask.
 17. The process ofclaim 11, wherein the cover includes an upper portion and a lowerportion, wherein the lower portion is placed below the projectionelectron lithography mask and the upper portion is placed above theprojection electron lithography mask.
 18. The process of claim 17,wherein the upper portion includes a handle for facilitating placementand alignment of the upper portion, the lower portion and the projectionelectron lithography mask and a protective portion that protects theactive region of the projection electron lithography mask.
 19. Theprocess of claim 18, wherein the protective portion includes a edgewhich contacts an outline on the projection electron lithography mask,wherein the edge is the only area of the upper portion to contact theprojection electron lithography mask.
 20. The process of claim 18,wherein the lower portion and the handle are shaped to facilitateplacement and alignment of the upper portion, the lower portion and theprojection electron lithography mask.